Many electrical weighing scales use electrical sensors that are coupled to amplifiers and provide an electrical signal indicative of a weight applied to the scale. In particular, many prior art approaches use a Wheatstone bridge arrangement of resistive sensors coupled to an amplifier. Such prior art scales are exemplified by, U.S. Pat. No. 4,739,848, entitled SCALES, issued to Tulloch; U.S. Pat. No. 4,800,973, entitled, PORTABLE ELECTRONIC SCALE OF MINIMAL THICKNESS AND WEIGHT, issued to Angel; and U.S. Pat. No. 4,880,069,entitled ELECTRONIC BATHROOM SCALE, issued to Bradley.
Inaccurate readings in electrical weighing scales may result from poor sensor tolerances and other disparities among the individual sensors. For example, if two piezoresistive sensors used in the same scale have different no-load resistances a false weight may be calculated. To compensate for sensor variations, the prior art has employed parallel resistance circuits such as that as set forth in U.S. Pat. No. 4,800,973,issued to Angel, and resistive balancing circuits such as that set forth in U.S. Pat. No. 3,266,584,entitled VEHICLE WEIGHING SCALE WITH OVERLAPPED LOAD BEARING PLATES, issued to Lee.
The manufacturing costs of previous scales frequently include the expenses of purchasing amplifiers, mounting amplifiers, and testing amplifiers for the proper amplification and summing of sensor signals.
Weighing scales are often placed in bathrooms and other places where temperature and humidity change rapidly. The performance of weighing scales may therefore deteriorate because of exposure to environmental conditions such as moisture, or contaminants such as household cleaning agents. Exposure to moisture and contaminants may cause inaccurate weight measurements over time because the resistance of circuit traces and component leads may be changed by corrosion.
Variations in temperature are one of the most common causes of inaccurate readings from piezoresistive elements. Since few places in a domestic setting vary in temperature as much as a bathroom, bathroom scales using piezoresistive elements must somehow compensate for temperature variations. The prior art is replete with various circuits and other schemes that provide temperature compensation to piezoresistive elements. Such prior art is exemplified by U.S. Pat. No. 4,333,349 to Mallon et al, entitled BINARY BALANCING APPARATUS FOR SEMICONDUCTOR TRANSDUCER STRUCTURES and U.S. Pat. No. 4,192,005 to Kurtz, entitled COMPENSATED PRESSURE TRANSDUCER EMPLOYING DIGITAL PROCESSING TECHNIQUES.
It is therefore, an objective of the present invention to provide a scale that is highly reliable yet is economical to manufacture.
It is a further objective of the present invention to provide a scale that is compensated for changes in environmental variables, such as temperature.
It is a further object of the present invention to provide a scale that compensates for use variables, such as the distribution of weight upon the scale.